This invention relates in general to roller bearings, and more particularly to a process for deriving contact geometry for the raceways and rollers of such bearings, with that geometry being weighted to account for the entire load cycle to which the bearing is subjected, to maintain the desired angular relationships between the raceways and rolling elements, and to maximize fatigue life.
The tapered roller bearing possesses the load carrying capacity of the cylindrical roller bearing while at the same time affording the capacity for adjustment and the accompanying precision that are characteristic of the angular contact ball bearing. In a simple single row tapered roller bearing, the cone (inner race) and cup (outer race) have tapered raceways, which are presented opposite to and face each other, and between the two raceways is a single row of tapered rollers, the frusto-conical side faces of which contact the raceways, thereby establishing lines of contact which converge toward a common region along the axis of rotation. The extended line contact enables the bearing to carry substantial loads, much like a cylindrical roller bearing, while the tapered geometry affords the capacity for adjustment. Like an angular contact ball bearing, this adjustment is achieved simply by shifting one race axially relative to the other. Indeed, a tapered roller bearing may be adjusted to a condition of preload where no axial or radial free motion exists between its cone and cup, and this renders the bearing suitable for use in precision machinery where an axis of rotation must remain perfectly stable with respect to some fixed reference.
The tapered rollers of course roll along the raceways of the cup and cone, and this rolling should occur without any sliding, that is to say, pure rolling contact should exist between the raceways and the rollers. This is achieved by constructing the raceways such that the two, if extended to their respective apexes, will have those apexes located generally at a common point along the axis of rotation for the bearing. The rollers, being in line contact with the raceways, will likewise, if extended to their respective apexes, have those apexes located at the very same point. A bearing so constructed is said to be "on taper" or "on apex".
Over the years the tapered roller bearing has undergone refinements, and as a result of such refinements it cannot be said that the raceways and rollers are "on taper" in the strictest sense of that expression. For one, it was discovered that where true on taper line contact exists with rollers having abrupt ends, high edge stresses occur at the ends of the line of contact. To eliminate the high edge stresses, the ends of the roller side faces or the raceways themselves are dubbed or rounded. Moreover, the raceways, or the roller side faces, and usually both, are "profiled", that is to say they are slightly crowned. This shifts more of the load to the midportions of the rollers and reduces the stress at the ends of the line of contact. According to one analytical procedure, a roller side face and body under load is divided into numerous elements, and for each element a diameter is calculated which will produce a stress that is essentially the same as the stress at the remaining elements. In other words, the calculations result in diameters which produce uniform stress along the lines of contact, and from these diameters one of course derives the profile. See U. S. Pat. 4,456,313.
A load placed on a machine component will cause that component to deflect, and where the component is supported on a tapered roller bearing, the deflection often distorts the bearing, creating a tilt or misalignment which concentrates the load at one end or the other of those rollers that are in the load zone, which is the sector of the bearing through which the load is transmitted. To compensate for misalignment, the raceway along which it manifests itself may be ground or otherwise configured off taper. Thus, when the load is applied, the raceway more closely approaches an on taper position, at least within the load zone. Since misalignment tends to concentrate the load at one end or the other of the row of rollers, a crown on the rollers or raceways will enable the bearing to better accommodate misalignment--and indeed this is a conventional procedure--but a high crown concentrates the stresses in a somewhat confined region between the ends of the rollers when no or less misalignment exists. This results in relatively high stresses over the reduced length of contact and thus decreases the fatigue life under lighter loads. See U.S. Pat. 1,794,580 and U.S. Pat. No. 3,95l,4S3.
In another approach, which also requires dividing the roller into elements for purposes of analysis, the diameter for each element is designed to provide it and the portions of the raceways along which it rolls with a life that corresponds to the life at the remaining elements. In short, this procedure produces uniform life which could result in a profile different from that required for uniform stress. Indeed, stress is one of the factors considered in arriving at the uniform life profile along with misalignment. In any event, the resulting profile is symmetrical about its center. See SAE Paper 850764, M. R. Hoeprich, Numerical Procedure for Designing Rolling Element Contact Geometry as a Function of Load Cycle, April 1985. Of all the design procedures, only this one uses more than a single design load. Even so, the best composite geometry is not obtained because the resulting profiles are symmetrical and on taper.
Heretofore, the general practice has been to provide tapered roller bearings with symmetrical profiles along their raceways and rollers, but not to compensate for misalignment by adjusting the raceway angle. The absence of raceway angle compensation for misalignment resides in part, from a fear of having a bearing so compensated perform poorly at loads other than that for which it is compensated. If the lines of contact along raceways were truly linear, this reasoning might have some justification, but the raceways and rollers where they contact each other are convex, and this keeps the region of contact toward the middle of the races, even at relatively light loads.
The present invention resides in a process for deriving the contact geometry and profiles of the raceways and roller side faces as well as raceway angle modification in a tapered roller bearing for which the load cycle and relative misalignment of the inner and outer races are known. By properly weighting the operating loads and misalignments that a tapered roller bearing will experience, the process produces a contact geometry which extends the life of the bearing well beyond that which could be expected from traditional modifications to a true "on taper" design. The process also has utility in connection with cylindrical roller bearings.